Satisfactory engine governing requires that the relationship of fuel delivery versus pump control rack or lever position be known. Ideally, the quantity of fuel delivered per stroke of an engine would increase linearly with rack position. However, unit fuel injectors for internal combustion engines which utilize a needle check type valve experience an undesirable change in the quantity of fuel delivered per stroke as the rack position changes. A typical fuel delivery curve is depicted by the upper curve shown in FIG. 1. The change in slope or knee in the fuel delivery versus rack position curve results in a portion of the curve having increased sensitivity to rack position. A minor change in rack position results in a large change in the fuel delivered to the cylinder.
This increased sensitivity causes difficulty controlling the speed of a diesel engine at low load or idle conditions. This results in "hunting" or "wandering" of the engine speed. Such engine speed instability results in difficulty maneuvering vehicles, difficulty controlling processes powered by the engine, failure of drive line components, and frequency variation of generator applications. The increased sensitivity to rack position can also make it difficult to match the fuel delivery of a set of fuel injectors. This can lead to uneven power distribution among the cylinders of an engine and can result in engine misfires.
The present invention is directed to overcome one or more of the problems as set forth above. The present invention provides a means whereby: (1) the knee of the fuel delivery versus rack position curve is lowered; and (2) the slope of the curve below the knee is made less steep without significantly affecting the fuel delivery at rack positions above the knee of the curve. The improved fuel delivery versus rack position curve is graphically depicted by the lower curve shown in FIG. 1.